1. Field of the Invention
The present invention pertains to the extraction of minerals from the earth and is especially adapted for the mining of coal. The need for sources of energy has multiplied rapidly over the past several decades as technology has developed and society has become increasingly mechanized. At the present time almost all of our energy needs are being met by combustion of various forms of fossil fuels such as coal, natural gas, and petroleum. Our increased energy needs dictate that the means for extracting these fuels from the earth be improved and made as efficient as possible. At the same time, it is imperative that deleterious effects on the environment due to such extraction be minimized.
Coal normally is found deposited in seams or layers interspersed with layers or strata of rock, soil, or other earth formation. The best method of mining would involve removing as much coal as possible from these layers with minimal removal of the surrounding non-coal strata and minimal disturbance or defacing of the terrain in the area. It is also necessary, where coal has been removed from a seam without disturbing the surface terrain, to leave adequate supporting structure in the seam to prevent cave-ins. Often a preliminary excavation is made, either from the surface or underground, to expose a wall containing one or more seals of coal. The coal is then removed by working from the wall into the seam.
Hand mining techniques in which the miners must enter the area of the seam as it is being mined are obviously dangerous and slow. Additionally, these techniques result in a great deal of waste if the coal seam is thin since enough material must be removed to allow the miners to enter even if much of the material is non-coal earth formation.
For these reasons, attention has been directed to the development of remote control apparatus for removing coal from a seam without the need for workers entering the seam. In particular there is need for such apparatus which is adaptable for the mining of thin seams.
2. Description of the Prior Art
One approach to solving this problem involved drilling into the seam from the exposed wall with an auger type drill. In some cases the leading end of the auger was provided with cutting means so that the auger itself served only as a conveyor for the cuttings and a transmitting member for the drilling thrust and torque, see e.g. U.S. Pat. No. 2,948,520. By using an auger of sufficiently small diameter, it was possible to remove coal from a thin seam without cutting into the surrounding rock. However, the only way of adapting the machine for mining a seam of another thickness was to replace the auger by another one of different diameter or to provide radially extensible auxiliary cutters. Furthermore, the diameter of the auger and/or cutters limited the width as well as the height of the hole which could be bored. This was less than satisfactory because the coal seams are usually substantially continuous in lateral extent.
Several attempts have been made to devise a machine which could remove more coal from the seam in a single pass by drilling a hole which was wider than the height of the seam. Two such machines are shown in U.S. Pat. No. 3,746,110 and British Pat. No. 800,864. Each of these machines has a cutting head which is wider than it is high. Thrust is transmitted to the cutting head by two parallel strings of rods in the machine of the British Patent and by two or more strings of auger segments, much the same as rods, in the machine of the U.S. Patent. Both of these machines suffer from the same basic disadvantages. In an effort to better distribute the drilling thrust across the cutting head, two or more strings of rods or the like are used. However, this introduces new problems of keeping these strings aligned and the forces balanced. Even though the two strings may be tied together laterally at a relatively few spaced locations along their lengths, they still act essentially as two separate strings and this alignment and balance is difficult, if not impossible, to achieve.
Another difficulty is that the strings of rods or auger segments are flexible under the high forces necessary for drilling. They tend to buckle, whip, etc. This makes the machine generally unstable, susceptible to failure of various parts, and difficult to control and steer from a remote location outside the hole being drilled. This tendency toward whipping and the like is evidenced by the fact that the rod strings of the machine of the British Patent are provided with upstanding guides which brace the strings against the roof of the hole to combat the whipping action.
Still another problem with these two machines is that the conveyors are unprotected against any material which might fall from the roof of the hole being drilled. This can jam the conveyor and in some cases can even result in the entire machine being caught in the hole. In this situation the expensive cutting head is either lost entirely, or it must be forcefully pulled from the hole resulting in severe damage to various parts of the machinery, particularly the conveyor. In the machine of the U.S. Patent, the conveyors are subjected to even worse abuse by virtue of the fact that they are the sole means of transmitting the drilling thrust and torque to the cutting head.